On 27 January 2009 the last six tank wagons of a ten-wagon dangerous goods train, carrying a mixed consignment of gas oil, diesel and kerosene to a fuel depot south of Kilmarnock, derailed as the train crossed a metal underbridge south of Stewarton, Ayrshire. The bridge, which takes the railway over the A735 road, collapsed and the derailed wagons overturned, coming to rest at various positions to the south of the bridge. Fuel from four of the derailed wagons leaked into the local environment and watercourses, and there were localised fires. There were no fatalities or injuries.
The immediate cause of the derailment was the collapse of the bridge (Bridge 88) that followed the catastrophic structural failure of its east and centre main girders. Heavy corrosion had so significantly weakened these main girders that they were no longer able to carry the loading from trains that were permitted to run over the bridge.
The causal factors were: the form of construction meant there was a hidden corrosion trap that affected the inner surfaces of the main girders; the corrosion resulted in a loss of thickness of the web plates of these girders allowing holes to form; and the use of incorrectly assumed dimensions for the thicknesses of these web plates in the last two routine assessments meant the reports of corrosion defects were not acted upon.
RAIB has made twelve recommendations concerning checks and intervention action on bridges; improvements to the management of the information used for making decisions about the structural safety of bridges and the structures assessment procedure; evaluation of the use of track geometry recordings as a means of identifying structural issues with railway underbridges; improvements to the construction of existing tank wagons registered in Great Britain; and evaluation of improvements to international regulations concerning tank wagon design.
Response to recommendations:
- RAIB will periodically update the status of recommendations as reported to us by the relevant safety authority or public body
- RAIB may add comment, particularly if we have concerns regarding these responses.